As known per se, a crystal laser comprises an optical cavity having a doped crystal housed therein, usually in the form of a bar, used as an amplifying medium under the effect of an optical pumping.
A high-power laser generally implies a significant heating of the crystal due to the low efficiency of the optical pumping. Now, for certain applications, such as for example X-ray imaging, it is necessary to have as temperature-stable an amplification as possible. Further, it can be observed that the optical pumping efficiency is optimal when the crystal temperature is in the range from 90 K to 200 K, which temperature range is difficult to reach without taking specific precautions relative to the crystal temperature.
Thereby, systems for cooling the crystal of a laser have been considered.
According to a first technique, the crystal bar is cut into segments and a cold gas is circulated between the bar segments. Such a solution can only be envisaged for very large lasers due to the size of the cooling system.
According to a second technique, the crystal is maintained in contact with a cooled support. Now, the crystal is held on the support by means of glue, of grease, or of indium, that is, a material which has a large thermal expansion coefficient difference with the crystal usually used in a laser. Under the effect of the crystal heating caused by the optical pumping, large expansion differences between the crystal and the material used to maintain it can be observed, such expansion differences ending up weakening the holding material and causing the separation of the crystal from the support, and thus causing in the end a malfunction of the cooling system.